Cold Electron Gas Research Articles

Revealing Optical Transitions and Carrier Recombination Dynamics within the Bulk Band Structure of Bi2Se3.

Bismuth selenide (Bi2Se3) is a prototypical 3D topological insulator whose Dirac surface states have been extensively studied theoretically and experimentally. Surprisingly little, however, is known about the energetics and dynamics of electrons and holes within the bulk band structure of the semiconductor. We use mid-infrared femtosecond transient reflectance measurements on a single nanoflake to study the ultrafast thermalization and recombination dynamics of photoexcited electrons and holes within the extended bulk band structure over a wide energy range (0.3 to 1.2 eV). Theoretical modeling of the reflectivity spectral line shapes at 10 K demonstrates that the electrons and holes are photoexcited within a dense and cold electron gas with a Fermi level positioned well above the bottom of the lowest conduction band. Direct optical transitions from the first and the second spin-orbit split valence bands to the Fermi level above the lowest conduction band minimum are identified. The photoexcited carriers thermalize rapidly to the lattice temperature within a couple of picoseconds due to optical phonon emission and scattering with the cold electron gas. The minority carrier holes recombine with the dense electron gas within 150 ps at 10 K and 50 ps at 300 K. Such knowledge of interaction of electrons and holes within the bulk band structure provides a foundation for understanding how such states interact dynamically with the topologically protected Dirac surface states.

On electron–proton energy exchange in strong magnetic field

Heating of protons in cold electron gas in strong magnetic field is studied. Calculations of heating process are preformed using molecular dynamics method. Estimations of heating rate depending on initial proton energies and electron gas temperatures are made.

Classical relativistic model for spin dependence in a magnetized electron gas

The response of a cold electron gas is generalized to include the spin of the electron described by the relativistically correct quasiclassical Bargmann-Michel-Telegdi (BMT) equation. The magnetization of the electron gas is assumed to be along the background magnetic field B and the spin-dependent contribution to the response tensor is proportional to the magnitude of the magnetization. The dispersion equation is shown to be quadratic in the refractive index squared, and dispersion curves for the two wave modes are plotted for cases where the magnetic field associated with magnetization is comparable with B. Two intrinsically spin-dependent wave modes are identified: one bounded by two resonances and the other by two cutoffs. The counterpart of the z mode can escape without encountering a resonance or a cutoff.

Bare Quark Stars or Naked Neutron Stars? The Case of RX J1856.5−3754

In a cool neutron star (T 106 K) endowed with a rather high magnetic field (B 1013 G), a phase transition may occur in the outermost layers. As a consequence, the neutron star becomes bare, i.e., no gaseous atmosphere sits on the top of the crust. The surface of a cooling, bare neutron star does not necessarily emit a blackbody spectrum because the emissivity is strongly suppressed at energies below the electron plasma frequency, ωp. Since ωp ≈ 1 keV under the conditions typical of the dense electron gas in the condensate, the emission from a T ~ 100 eV bare neutron star will be substantially depressed with respect to that of a perfect Planckian radiator at most energies. Here we present a detailed analysis of the emission properties of a bare neutron star. In particular, we derive the surface emissivity for an Fe composition in a range of magnetic fields and temperatures representative of cooling isolated neutron stars, like RX J1856.5-3754. We find that the emitted spectrum is strongly dependent on the electron conductivity in the solid surface layers. In the cold electron gas approximation (no electron-lattice interactions), the spectrum turns out to be a featureless depressed blackbody in the 0.1-2 keV band with a steeper low-energy distribution. When damping effects due to collisions between electrons and the ion lattice (mainly due to electron-phonon interactions) are accounted for, the spectrum is more depressed at low energies and spectral features may be present, depending on the magnetic field strength. Details of the emitted spectrum are found, however, to be strongly dependent on the assumed treatment of the transition from the external vacuum to the metallic surface. The implications of our results for RX J1856.5-3754 and other isolated neutron stars are discussed.

Ultrafast dephasing in strongly correlated systems

AbstractUsing projection techniques we derive the equation of motion for the third‐order nonlinear optical polarization of a strongly correlated system. We do not assume a Hartree–Fock ground state, but include the ground state correlations via time‐independent interaction parameters and energies. Our theory can be used to describe the ultrafast nonlinear optical response of a strongly correlated cold electron gas, such as the two‐dimensional electron gas in the quantum Hall effect regime. For this system, we show that the collective electronic excitations lead to a time‐dependent coupling of the Landau level magnetoexcitons and a strong non‐Markovian exciton dephasing. We discuss the signatures in the transient four‐wave‐mixing spectrum.

Exciton-Electron Scattering in Semiconductor Microcavities: Tool for Polariton Lasing

We show theoretically that the introduction of a cold electron gas into quantum microcavities induces efficient electron-polariton scattering. This process allows the condensation of the polaritons accumulated at the bottleneck to the final emitting state with a transition time of a few picoseconds, opening the way to a new generation of low-threshold light-emitting devices.

Polariton lasing by exciton-electron scattering in semiconductor microcavities

The relaxation bottleneck present in the dispersion relation of exciton polaritons in semiconductor microcavities has prevented the realization of low threshold lasing based on exciton-polariton condensation. Here we show theoretically that the introduction of a cold electron gas into such structures induces efficient electron-polariton scattering. This process allows the condensation of the polaritons accumulated at the bottleneck to the final emitting state with a transition time of a few picoseconds, opening the way to a new generation of low-threshold light-emitting devices.

Polariton Lasing Due to the Exciton-Electron Scattering in Semiconductor Microcavities

The issue of polariton lasing in n-doped microcavities is addressed. We show theoretically that the introduction of a cold electron gas into quantum microcavities induces efficient electron–polariton scattering. This process allows the condensation of the polaritons accumulated at the bottleneck to the final emitting state with a transition time of a few picoseconds, opening the way to a new generation of low-threshold light emitting devices

Polariton Lasing Due to the Exciton–Electron Scattering in Semiconductor Microcavities

The issue of polariton lasing in n-doped microcavities is addressed. We show theoretically that the introduction of a cold electron gas into quantum microcavities induces efficient electron–polariton scattering. This process allows the condensation of the polaritons accumulated at the bottleneck to the final emitting state with a transition time of a few picoseconds, opening the way to a new generation of low-threshold light emitting devices.

Wave propagation in strongly coupled quasi-one-dimensional quantum plasma in a magnetic field

A cold electron gas fills the lowest Landau level for superstrong magnetic fields and very low densities. In such cases, in general, the potential energy of the particles is equal to or greater than their kinetic energy (strongly coupled plasmas), and a special approach is called for. The STLS (Singwi, Tosi, Land, and Sjolander) approximation scheme is used to study the dispersion and damping of the low-frequency modes, i.e., the whistler and the extraordinary modes for zero temperature. The lowest order dispersion for all modes in consideration are unaffected by correlations, but for undamped plasmas the correlation term is of the order /gamma/a/sup 2/c/sup /minus/2/. Further, /delta/(/omega//sup 2/) for the whistler mode becomes infinite at /gamma/ = 3; its behavior critically depends on the filling number /eta//sub Fc/ = /epsilon//sub F///Dirac h//Omega/, where /epsilon//sub F/ is the Fermi energy and /Omega/ is the electron cyclotron frequency.

Wave propagation in quasi-one-dimensional quantum plasma in a magnetic field

A cold electron gas fills the lowest Landau level for high enough magnetic fields and for low enough densities. Such a situation is expected to occur for the Malmberg-O'Neil experiment and also for pulsar crusts and atmospheres. Such plasmas behave as a quasi-one-dimensional system and exhibit some peculiarities in their wave structure. We study the dispersion and damping of the low frequencies, i.e., the whistler mode, and the extraordinary mode for zero temperature. The behavior of the whistler mode depends critically on the “filling number”ηFc=eF/ ħΩ, where eF is the Fermi energy and Ω is the cyclotron frequency. The one-dimensional character of the system affects the pair excitation spectrum and thus the decay of modes. We find that, in contrast to the three-dimensional situation, the plasma mode and the extraordinary mode remain undamped, while the whistler mode is undamped for all but very highk values.

Scattering of spatially homogeneous isotropic radiation by a cold electron gas

Multiple Compton scattering by a gas of fixed electrons is considered. It is assumed that the photon energy is much less than the electron rest-energy. Exact solutions are obtained for the differential equation describing such scattering in the linear approximation and the dispersionless approximation. The evolution of the radiation spectrum in a special case is followed. 14 references.

Boltzmann-Equation Study of Hot-Electron Relaxation in Metals

The relaxation of a hot-electron distribution, in strong interaction with a dense cold-electron gas, is studied using a time-dependent Boltzmann-equation approximation. In the approximation which drops a collision recovery term, the theory becomes equivalent to Quinn's self-energy approach. Assuming the distribution function to be isotropic in momentum space for all times, the Boltzmann equation is integrated to give the time development of an initially sharply peaked distribution. Attention is focused on the shape of the distribution as it relaxes, and the effect of the spreading of the distribution on the rate of energy loss is analyzed. In the final section of the paper, the results are discussed with relation to certain models of hot-electron transport in thin-film structures.